Spark-ignited engine NOx emissions in a low-nitrogen oxycombustion environment

• We performed an experiment operating an IC engine on methane-oxycombustion.
• We used two models to predict burned-gas temperatures and NOx formation.
• NOx emissions in oxycombustion vary linearly with N2 volume concentration.
• CO2 working fluid produces higher thermal efficiency and lower NOx than CO2 and H2O.
• NOx mechanisms tuned for air accurately predict emissions in low N2 environments.

This paper investigates the formation of the pollutant nitric oxides (NOx) in the low-nitrogen (N2) environment of methane oxycombustion in a spark-ignited (SI) internal combustion engine. Working fluid composition, N2 concentration, O2 concentration, compression ratio (CR) and spark-timing have been investigated to evaluate the feasibility of operating such a system below NOx regulation levels without after-treatment systems.NOx emissions in g/kW h are shown under equivalent CR, intake temperature, and indicated mean effective pressure (IMEP) at maximum brake torque spark-timing, to have an approximately linear dependence on N2 concentration from no N2 to normal air combustion. At a given N2 concentration, NOx emissions were found to be adversely correlated with power, thermal efficiency, and the coefficient of variation of IMEP. It was found that with 2–3% N2 by volume in the working fluid, it was possible to reduce NOx emissions to satisfy regulation levels, but this corresponds to non-ideal engine performance in other metrics. Satisfying regulations while operating at the maximum thermal efficiency required the N2 concentration be reduced to 1–2% by volume.The system was simulated using an AVL Boost model, with results indicating that the increasing NOx concentrations at higher O2 cases and earlier spark-timings can largely be attributed to higher burned-gas temperatures. An additional simulation utilizing CHEMKIN and the GRI 3.0 mechanism was used to estimate NOx formation, and with results indicating that air-calibrated NOx mechanisms maintain reasonable accuracy in low-N2 environments.